The present invention relates to a bidirectional optical semiconductor apparatus, and more particularly relates to a bidirectional optical semiconductor apparatus with enhanced optical isolation performance.
In recent years, a fiber-to-the-user system has been proposed for transmitting data from a base station to home users by way of optical fibers. In the fiber-to-the-user system, optical communication is established bidirectionally by providing optical transmitters and receivers for both the base station and the home users. Accordingly, an optical transmitter/receiver apparatus, including: an optical trans mitter; an optical receiver; and an optical transmission line, is required. The optical transmitter may be a semiconductor light-emitting device for outputting an optical signal. The optical receiver may be a semiconductor light-receiving device like a photodiode for receiving the optical signal. And the optical transmission line may be a bundle of optical fibers for connecting the optical transmitter and receiver together.
In a prior art optical transmitter/receiver apparatus, an optical fiber for an optical transmitter, including a semiconductor laser device assembled in a package, is coupled to another optical fiber for an optical receiver, including a light-receiving device assembled in another package, via a coupler. Hereinafter, this apparatus will be called a xe2x80x9cfirst conventional optical transmitter/receiver apparatusxe2x80x9d for convenience of explanation.
Although commercially available optical transmitter and receiver can be used for this apparatus, such an apparatus is disadvantageous in view of downsizing and cost reduction, because separate optical transmitter and receiver should be coupled together via a coupler.
In order to solve such a problem, another prior art optical transmitter/receiver apparatus has a planar lightwave circuit (PLC) structure. The apparatus includes: a quartz substrate; semiconductor laser and light-receiving devices integrally supported by the substrate; and an optical waveguide formed within the substrate. Hereinafter, this apparatus will be referred to as a xe2x80x9csecond conventional optical transmitter/receiver apparatusxe2x80x9d.
In the second conventional optical transmitter/receiver apparatus, downsizing is realized to a certain degree by integrating the semiconductor laser and light-receiving devices on a single quartz substrate. However, since the area of the optical waveguide formed within the substrate can be no smaller than a certain limit, the size of the apparatus still cannot be regarded as sufficiently small. In addition, since the PLC should be connected to optical fibers, the cost effectiveness thereof is not totally satisfactory, either.
Thus, the present inventors proposed a bidirectional optical semiconductor apparatus, such as that shown in FIG. 9, in PCT International Application No. WO97/06458. Specifically, a silicon substrate 2 is placed on the bottom of a package 1 on the left-hand side in FIG. 9. And a semiconductor laser device (i.e., an exemplary semiconductor lightemitting device) 3, for emitting xe2x80x9csignal lightxe2x80x9d at a wave 10 length of 1.3 xcexcm, for example, is fixed onto the upper surface of the silicon substrate 2. In this specification, the xe2x80x9csignal lightxe2x80x9d means an optical signal, which is output from an optical transmitter, propagated through an optical waveguide and then received by an optical receiver in the form of light all through these processes. In the following description, the xe2x80x9csignal lightxe2x80x9d in this sense will be simply referred to as xe2x80x9clightxe2x80x9d, unless explicitly stated otherwise. On the right-hand side in FIG. 9, a glass substrate (i.e., an exemplary substrate) 4, having a groove extending in the direction of the optical axis, is placed on the bottom of the package 1. An optical fiber 5 (i.e., an exemplary optical waveguide) is embedded within the groove of the glass substrate 4 and one end of the optical fiber 5, closer to the laser device, is fixed onto the upper surface of the silicon substrate 2. A half mirror (i.e., an exemplary optical branching filter) 6 is inserted into the glass substrate 4 such that a predetermined angle is formed between the mirror 6 and the optical axis. The half mirror 6 transmits about 50% of the output light, incoming from the left of the optical fiber 5 at the wavelength of 1.3 xcexcm, and reflects upward about 50% of the input light, incoming from the right of the optical fiber 5 also at the wavelength of 1.3 xcexcm. On the upper surface of the glass substrate 4, a light-receiving device 7 such as a photodiode (i.e., an exemplary semiconductor light-receiving device) of a surface-receiving type is fixed for receiving the input light, reflected by the half mirror 6, and outputting photocurrent.
Since the substrate embedding the optical waveguide, the semiconductor light-emitting device and the optical branching filter are integrated within a single apparatus, the bidirectional optical semiconductor apparatus is advantageous in improving the ease of use and reducing the overall size thereof. However, this bidirectional optical semiconductor apparatus still has a problem regarding optical isolation. Specifically, since the light, emitted from the semiconductor light-emitting device, is partially received by the semiconductor light-receiving device, optical isolation adversely deteriorates.
During a relaxation time of certain duration immediately after the semiconductor light-emitting device has been driven with a pulse, the output light, emitted from the semiconductor light-emitting device, leaves some tailing light behind. Thus, with the light-emitting and light-receiving ends insufficiently isolated optically, when the input light is re ceived by the semiconductor light-receiving device, the tailing component of the output light, emitted from the semiconductor light-emitting device, is also received by the receiving device to form noise with respect to the incoming light. As a result, the performance of the semiconductor light-receiving device deteriorates. Accordingly, in performing bidirectional optical communication, improvement of optical isolation plays a crucial role.
In the first conventional optical transmitter/receiver apparatus, deterioration in optical isolation is allegedly suppressed by coupling the optical fiber on the transmitting end to the optical fiber on the receiving end with an offset provided at the coupler.
In contrast, no measures have been taken to suppress the deterioration in optical isolation in the second conventional optical transmitter/receiver apparatus and in the bidirectional optical semiconductor apparatus disclosed by the present inventors in the above-identified patent application.
An object of the present invention is reducing the overall size and improving the optical isolation of a bidirectional optical semiconductor apparatus, in which a substrate with an optical waveguide, a semiconductor light-emitting device, an optical branching filter, and a semiconductor light-receiving device are integrated.
As will be described later, the substrate embeds an optical waveguide, through which output light and input light are propagated. The semiconductor light-emitting device emits the output light toward one end of the optical waveguide. The optical branching filter is provided for the optical waveguide for transmitting at least part of the output light and guiding at least part of the input light to the outside of the optical waveguide. And the semiconductor light-receiving device is provided over the substrate for receiving the input light guided by the optical branching filter to the outside of the optical waveguide.
Next, the optical isolation between the light-emitting and light-receiving ends in such a bidirectional optical semiconductor apparatus will be described based on the results of experiments carried out to analyze the reason why the isolation performance deteriorates, in particular.
First, specific results of experiments carried out to analyze the optical isolation between the light-emitting and light-receiving ends in the bidirectional optical semiconductor apparatus will be described. In the following description, output light, emitted from a semiconductor laser device 3 (i.e., an exemplary semiconductor light-emitting device) at a wavelength of 1.3 xcexcm, is supposed to be input at one end of an optical fiber 5 (i.e., as an optical waveguide). Also, input light, transmitted at a wavelength of 1.3 xcexcm, is supposed to be received at the other end of the optical fiber 5. And the half mirror 6 and photodiode 7 are supposed to be used as exemplary optical branching filter and semiconductor light-receiving device, respectively.
If the output light is emitted from the semiconductor laser device 3 at 10 mW, about 20% of the output light is coupled to the optical fiber 5. And about 50% of that portion of the output light, which has been coupled to the optical fiber 5, is transmitted through the half mirror 6 to be output to is the outside. That is to say, in this case, the light is output from the optical fiber 5 to the outside at about 1 mW.
If the output power of the output light, which has been emitted from the semiconductor laser device 3 and then output to the outside through the optical fiber 5 with a bias voltage of xe2x88x923 V applied to the light-receiving device 7, is 1 mW, then the photocurrent output from the light-receiving device 7 is 8.5 xcexcA on average. On the other hand, if input light of 1 mW is received through the optical fiber 5 with a bias voltage of xe2x88x923 V applied to the light-receiving device 7, then the photo-current output from the light-receiving device 7 is 450 xcexcA on average (i.e., average sensitivity is 0.45 A/W). Accordingly, the optical isolation between the light-emitting and light-receiving ends is 17 dB.
Next, it will be described based on the results of the experiments why the optical isolation between the light-emitting and light-receiving ends deteriorates in the bidirectional optical semiconductor apparatus.
As a result of the experiments, the present inventors spotted the root of deterioration in optical isolation at a diversity of paths (mainly three), through which the output light, emitted from the semiconductor laser device 3, passes and is ultimately received at the light-receiving device 7. In FIG. 10, the first input path is defined for some components of the output light of the semiconductor laser device 3, which are propagated through the cladding of the optical fiber 5 and then leak therefrom to reach the light-receiving device 7 without being reflected by the package 1. In this specification, the light passing through this path will be referred to as xe2x80x9ccladding leakage lightxe2x80x9d. The second input path is defined for other components of the output light of the semiconductor laser device 3, which are reflected by the half mirror 6 and then reflected again at random by the bottom of the package 1 to reach the light-receiving device 7. In this specification, the light passing through this path will be referred to as xe2x80x9chalf-mirror reflected lightxe2x80x9d. The third input path is defined for components of the output light emitted from the other end of the semiconductor laser device 3, opposite to the optical fiber 5, and then reflected at random by the side and upper surfaces of the package 1 to reach the light-receiving device 7. In this specification, the light passing through this path will be referred to as xe2x80x9cpackage scattering lightxe2x80x9d.
Based on these findings, the present invention provides various measures for preventing the cladding leakage light, half-mirror reflected light or package scattering light from being accidentally incident onto the semiconductor light-receiving device. Specifically, the present invention is implemented as a bidirectional optical semiconductor apparatus according to any of the following embodiments.
A first bidirectional optical semiconductor apparatus according to the present invention includes: a substrate embedding an optical waveguide, through which output light and input light are propagated; a semiconductor light-emitting device for emitting the output light toward one end of the optical waveguide; an optical branching filter, provided in the optical waveguide, for transmitting at least part of the output light and guiding at least part of the input light to the outside of the optical waveguide; a semiconductor light-receiving device, provided over the substrate, for receiving the input light guided by the optical branching filter to the outside of the optical waveguide; and a light-blocking member. formed on the surface of the semiconductor light-receiving device, for blocking the light emitted from the semiconductor light-emitting device.
The first bidirectional optical semiconductor apparatus includes a light-blocking member, formed on the surface of the semiconductor light-receiving device, for blocking the light emitted from the semiconductor light-emitting device. Accordingly, components of light, which have been emitted from the semiconductor light-emitting device and then directed toward the semiconductor light-receiving device without being propagated through the optical waveguide, are blocked by the light-blocking member on the surface of the receiving device and not incident onto the receiving device. As a result, the optical isolation improves.
A second bidirectional optical semiconductor apparatus according to the present invention includes: a substrate embedding an optical waveguide, through which output light and input light are propagated; a semiconductor light-emitting device for emitting the output light toward one end of the optical waveguide; an optical branching filter, provided in the optical waveguide, for transmitting at least part of the output light and guiding at least part of the input light to the outside of the optical waveguide; a semiconductor light-receiving device, provided over the substrate, for receiving the input light guided by the optical branching filter to the outside of the optical waveguide; and a light-blocking member, formed on the surface of the substrate, for blocking the light emitted from the semiconductor light-emitting device.
The second bidirectional optical semiconductor apparatus includes a light-blocking member, formed on the surface of the substrate, for blocking the light emitted from the semiconductor light-emitting device. Accordingly, components of light, which have been emitted from the semiconductor light-emitting device and then deviated from the optical waveguide, are blocked by the light-blocking member on the surface of the substrate and not incident onto the semiconductor light-receiving device. As a result, the optical isolation improves.
In the first or second bidirectional optical semiconductor apparatus, the light-blocking member is preferably a plastic film, a metal layer or a thin-film dielectric filter.
A third bidirectional optical semiconductor apparatus according to the present invention includes: a substrate embedding an optical waveguide, through which output light and input light are propagated; a semiconductor light-emitting device for emitting the output light toward one end of the optical waveguide; an optical branching filter, provided in the optical waveguide, for transmitting at least part of the output light and guiding at least part of the input light to the outside of the optical waveguide; a semiconductor lights-receiving device, provided over the substrate, for receiving the input light guided by the optical branching filter to the outside of the optical waveguide; and a highly reflective layer, provided on a surface of the substrate opposite to the semiconductor light-receiving device, for reflecting the light emitted from the semiconductor light-emitting device in a direction departing from the semiconductor light-receiving device at a high reflectivity.
The third bidirectional optical semiconductor apparatus includes a highly reflective layer, provided on a surface of the substrate opposite to the semiconductor light-receiving device, for reflecting the light emitted from the semiconductor light-emitting device in a direction departing from the semiconductor light-receiving device at a high reflectivity. Thus, the light, which has been emitted from the emitting device and deviated from the optical waveguide, is reflected by the highly reflective layer provided on the surface of the substrate opposite to the receiving device in a direction departing from the receiving device at a high reflectivity. And the light is not incident onto the semiconductor light-receiving device. As a result, the optical isolation improves.
A fourth bidirectional optical semiconductor apparatus according to the present invention includes: a substrate embedding an optical waveguide, through which output light and input light are propagated; a semiconductor light-emitting device for emitting the output light toward one end of the optical waveguide; an optical branching filter, provided in the optical waveguide, for transmitting at least part of the output light and guiding at least part of the input light to the outside of the optical waveguide; a semiconductor light-receiving device, provided over the substrate, for receiving the input light guided by the optical branching filter to the outside of the optical waveguide; and a light-absorbing member, provided over part of the substrate other than the optical waveguide, for absorbing the light emitted from the semiconductor light-emitting device.
The fourth bidirectional optical semiconductor apparatus includes a light-absorbing member, provided over part of the substrate other than the optical waveguide, for absorbing the light emitted from the semiconductor light-emitting device. Thus, the light, which has been emitted from the semiconductor light-emitting device and deviated from the optical waveguide, is absorbed by the light-absorbing member provided over part of the substrate other than the optical waveguide, and not incident onto the semiconductor light-receiving device. As a result, the optical isolation improves.
A fifth bidirectional optical semiconductor apparatus according to the present invention includes: a substrate embedding an optical waveguide, through which output light and input light are propagated; a semiconductor light-emitting device for emitting the output light toward one end of the optical waveguide; an optical branching filter, provided in the optical waveguide, for transmitting at least part of the output light and guiding at least part of the input light to the outside of the optical waveguide; a semiconductor light-receiving device, provided over the substrate, for receiving the input light guided by the optical branching filter to the outside of the optical waveguide; and an optical element, provided over part of the substrate other than the optical waveguide, for absorbing the light emitted from the semiconductor light-emitting device and deviated from the optical waveguide or for reflecting the light in a direction departing from the semiconductor light-receiving device.
The fifth bidirectional optical semiconductor apparatus includes an optical element, provided over part of the substrate other than the optical waveguide, for absorbing the light emitted from the semiconductor light-emitting device and deviated from the optical waveguide or for reflecting the light in a direction departing from the semiconductor light-receiving device. Thus, the light, which has been emitted from the semiconductor light-emitting device and deviated from the optical waveguide, is absorbed or reflected in a direction departing from the semiconductor light-receiving device by the optical element, and not incident onto the semiconductor light-receiving device. As a result, the optical isolation improves.
A sixth bidirectional optical semiconductor apparatus according to the present invention includes: a substrate embedding an optical waveguide, through which output light and input light are propagated; a semiconductor light-emitting device for emitting the output light toward one end of the optical waveguide; an optical branching filter, provided in the optical waveguide, for transmitting at least part of the output light and guiding at least part of the input light to the outside of the optical waveguide; a semiconductor light-receiving device, provided over the substrate, for receiving the input light guided by the optical branching filter to the outside of the optical waveguide; a package for housing the substrate, the semiconductor light-emitting device, the optical branching filter and the semiconductor light-receiving device; and a light-absorbing film, formed on an inner wall surface of the package, for absorbing the light emitted from the semiconductor light-emitting device.
The sixth bidirectional optical semiconductor apparatus includes a light-absorbing film, formed on an inner wall surface of the package, for absorbing the light emitted from the semiconductor light-emitting device. Thus, the light, which has been emitted from the semiconductor light-emitting device and deviated from the optical waveguide, is absorbed by the light-absorptive film on the inner wall surface of the package, and not incident onto the semiconductor light-receiving device. As a result, the optical isolation improves.
In one embodiment of the present invention, the substrate is preferably made of quartz glass, silicon crystals or polymers.
In another embodiment of the present invention, the optical branching filter is preferably an optical element provided to intersect with the optical waveguide.
In still another embodiment, the optical waveguide is preferably a core of an optical fiber embedded in a groove formed in the substrate.
In an embodiment where the optical waveguide is the core of the optical fiber, the optical fiber is preferably fixed by a fixing member to the substrate and embedded in the groove, and a refractive index of the fixing member is preferably smaller than a refractive index of a cladding of the optical fiber.
In another embodiment where the optical waveguide is the core of the optical fiber, the optical fiber is preferably fixed by a fixing member to the substrate and embedded in the groove. And a refractive index of the fixing member in a region thereof closer to the semiconductor light-receiving device is preferably smaller than a refractive index of a cladding of the optical fiber, while a refractive index of the fixing member in a region thereof distant from the semiconductor light-receiving device is preferably larger than the refractive index of the cladding of the optical fiber.
Then, the light, which has been emitted from the emitting device and then deviated from the core of the optical fiber, is confined in the optical fiber in the region closer to the receiving device and absorbed into the fixing member in the region distant from the receiving device. As a result, the optical isolation improves with much more certainty.
In still another embodiment where the optical waveguide is the core of the optical fiber, a refractive index of the substrate is preferably smaller than a refractive index of the cladding of the optical fiber.
In still another embodiment where the optical waveguide is the core of the optical fiber, a refractive index of the substrate in a region thereof closer to the semiconductor light-receiving device is preferably smaller than a refractive index of a cladding of the optical fiber, while a refractive index of the substrate in a region thereof distant from the semiconductor light-receiving device is preferably larger than the refractive index of the cladding of the optical fiber.
Then, the light, which has been emitted from the emitting device and then deviated from the core of the optical fiber, is confined in the optical fiber in the region closer to the receiving device and absorbed into the substrate in the region distant from the receiving device. As a result, the optical isolation improves with much more certainty.